The present industrial standard personnel dosimeter is a plastic compartment containing a monolithic chip (approximately 3 mm.times.3 mm.times.1 mm) of a thermoluminescent crystalline material; for example, lithium fluoride doped with titanium and magnesium (LiF:Ti,Mg). Dosimeters contain one or more compartments with a corresponding thermoluminescent material chip in each compartment.
The compartments are used to facilitate handling of the tiny chips, reducing the risk of loss of a chip, and providing a sufficiently large surface upon which identifying information may be recorded.
The chips are normally interrogated after removing them from their compartments by a process referred to as Thermoluminescent Dosimetry (TLD). The TLD process entails heating to a peak temperature then observing the light photons emitted as the temperature of the chips approaches the peak temperature. The number of photons are proportional to the level of radiation exposure. Of course, the chips themselves and any other material exposed to the peak temperatures must be able to withstand the peak temperatures. This requirement generally precludes the use of a composite dosimeter wherein the dosimeter chip material is held in a plastic matrix because most plastic matrices soften or degrade at the peak temperature.
Nevertheless, B. E. Bjarngard, R. C. McCall, and I. A. Berstein reported a composite dosimeter in their paper entitled LITHIUM FLUORIDE-TEFLON THERMOLUMINESCENCE DOSIMETERS, published April 1967 in the Proceedings of International Conference on Luminescence Dosimetry, at Stanford University on Jun. 21-23, 1965. Teflon or Polytetrafluoroethylene (PTFE) is one of the few polymers that can withstand the peak temperature necessary to stimulate phosphorescence of the thermoluminescent dosimeter material. Bjarngard et al. report sensitivity of the composite dosimeter from about 50 mR. Sensitivity is likely limited by the opacity of the PTFE, and/or background signal from the PTFE. Although not tested, use of clear PTFE as is now used as a fiber optic sheath might improve sensitivity of this dosimeter. However, the present cost of the clear PTFE is prohibitive for industrial dosimeter applications.
In addition, R. Bernhardt et al. in their paper RADIATION DOSIMETRY BY OPTICALLY STIMULATED PHOSPHORESCENCE OF CaF.sub.2 :Mn, August 1974 demonstrate optical stimulation of a CaF.sub.2 :Mn-Teflon (Teflon is a trade name for the chemical compound polytetrafluoroethylene) dosimeter for exposures from 1 to 10.sup.5 rad. The lower limit of 1 rad is reported as achievable using a measuring time of 0.5 min and a stimulation time of 0.1 min for a total time of 0.6 min. Shorter measuring times result in raising the lower limit, or reducing the sensitivity. The authors conclude that this dosimetric technique is appropriate for accident dosimetry.
A more recent method of interrogation was described in U.S. Pat. No. 4,954,707, to Miller et. al. entitled SYSTEM FOR USE WITH SOLID STATE DOSIMETER. In this patent, the method of interrogation is cooling of a solid state material chip and exposing the cooled chip to light for photoconversion of light sensitive traps, then permitting the chip to warm to ambient temperature wherein during the warming, photons are emitted from the light sensitive traps proportional to the level of radiation exposure. It is important to note that the photon emission occurs later in time than the photoconversion. This method is referred to as Cooled Optically Stimulated Luminescence (COSL). The COSL method is advantageous over the TLD method because the lower temperatures are less harmful to a wider variety of materials than the higher temperatures of the TLD process. The photoconversion in the COSL method as practiced using CaF.sub.2 :Mn or CaF.sub.2 :Dy is accomplished using ultraviolet (UV) light because the UV light produces the most photons per unit of ionizing radiation exposure thereby producing sensitivities on the order of about 1 milliRoentgen (mR).
A yet more recent method of interrogation was described in U.S. Pat. No. 5,272,348 to Miller, entitled METHOD FOR RADIATION DETECTION AND MEASUREMENT, wherein the dosimeter material is stimulated by light energy at one wavelength to produce photons at another wavelength at ambient temperatures. The emitted photons are proportional to the level of radiation exposure. This ambient temperature method is referred to as Optically Stimulated Luminescence (OSL). The OSL process has the advantage that no heating or cooling is necessary to obtain emitted photons. In the OSL process, photon emission occurs nearly simultaneously with light stimulation. The optical stimulation of the OSL process is practiced using light in the visible and infrared. It is preferred to use light at a wavelength that is near the excitation wavelength of the dosimeter material traps.
Regardless of the method of interrogation, however, the chips still required compartments for handling and interrogation independent of the compartment. The compartment plus chip cost along with the handling cost for interrogation made it prohibitive to perform large scale population dosimetry studies. Also, because of the cost of the chips, in industrial employee applications it was necessary to re-use the chips because the additional handling was less costly than replacement of the chips.